Where Do The Elements Come From?

Vic · October 29, 2018

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If you’ve ever had a chemistry class, or even just a basic science class, you have likely encountered the periodic table of elements. It is a list of all the atomic elements we know of, sorted by mass and several other properties.

Our bodies contain quite a few of these, such as hydrogen, oxygen, carbon, nitrogen, phosphorus, calcium, iron, and many more. Vulcans, of course, would have to add copper.

(a)

(b)

Figure 1: (a) Our current periodic table of the elements (90 naturally occurring elements and a bunch more short-lived ones produced in the lab, Credit: http://iupac.org. (b) In Star Trek’s time we’ll know a few more. Credit: Star Fleet Medical Reference Manual.

Have you ever wondered where all these different elements come from? The most-common element is the first one, hydrogen. It and some of the next two, helium and lithium, were produced during the Big Bang, the beginning of the expansion of our Universe. The dense, hot cores of stars produce some of the others via thermonuclear fusion. For example, our Sun has been fusing hydrogen to helium for the last five billion years and will continue to do so for the next five billion years. In a later stage of life our Sun will produce carbon from helium. More massive stars can produce heavier elements during their lifetimes (our Sun is a somewhat below average star). The heaviest element that the most massive star can produce is iron. The reason is that up to iron the fusion reactions produce excess energy (which is why stars shine). To produce elements heavier than iron you have to put energy into the fusion reaction, and nature doesn’t work that way unless there are vast amounts of energy in the ambient environment, a rare situation.

Many of us are wearing silver or gold jewelry (the vendors at Star Trek conventions have ever fancier versions of the various delta shields). So where do these heavier elements come from?

Once a massive star has reached the end of its fusion sequence, it explodes in a brilliant supernova. Such an event can be a billion times as bright as our Sun. This explosion releases a lot of extra energy into the locality, which can then be taken in by fusion of heavier elements. Until fairly recently, that’s where we thought all the heavier elements were created.

In 1915, Albert Einstein said that if electromagnetic radiation (i.e. light) is generated by moving charges or changing magnetic fields, then gravitational radiation should be generated by moving masses. Sounds pretty logical, right? It took us 100 years to prove him right.

Now, what does that have to do with producing gold and silver? The 2015 detection of gravitational waves was possible by observing two events produced by a pair of black holes and a pair of neutron stars. I’ll go more into detail about gravitational waves in the next blog, but for now, I’d like to concentrate on the binary neutron stars. As they orbit each other, their orbits get smaller (which is why they emit energy in the form of the gravitational waves), and eventually they will merge together. As neutron stars merge and explode in what is called a kilonova, their very dense substance suddenly becomes less dense, allowing for fusion reactions of heavy elements to take place in this environment with lots of extra energy. That’s yet another way of making elements heavier than iron.

Two merging neutron stars exploding in a kilonova

Figure 2: This is a graphic representation of two merging neutron stars exploding in a kilonova. Credit: npr.org

According to Edo Berger from the Harvard-Smithsonian Center for Astrophysics, the collision of two neutron stars may produce 10 times the mass of our Moon in pure gold (or more, and other cool stuff like silver and platinum). So, if it’s gold you want, you know where to go with your starship (wait to approach until after the kilonova, as you do not want to get caught in that sort of cataclysm).

If we redo our table of the elements a bit, we can color-code where all the different elements come from.

The periodic table of the elements color-coded by origin